In the case of electrically doped layers, the production of such layers requires the deposition of materials, host and dopant, which are supposed to undergo a chemical reaction in the layer. This holds both for p- and n-type doping. More precisely, a redox reaction occurs between host and dopant molecules and results in an at least partial charge transfer, thereby providing additional free charge carriers which increase the electrical conductivity compared to the non-doped host material. Electrically doped layers find their main application so far in the production of PIN devices, such as PIN OLEDs (OLED—Organic Light Emitting Diode) or PIN solar cells. PIN is the abbreviation for a stack consisting of a p-doped zone on an intrinsic zone on a n-doped zone. For other applications the intrinsic zone is skipped resulting in a PN junction.
The use of electrically doped structures helps to improve the device parameters of OLEDs, such as low power consumption, long lifetime, extended material and thickness flexibility. The general structure of a PIN OLED is the following:    a) an anode,    b) a hole transport layer applying p-doping (HTL),    c) an intrinsic zone comprising an emitting layer (EML), and optionally an electron blocking layer (EBL) towards the HTL and a hole blocking layer (HBL) towards the ETL,    d) an electron transport layer applying n-doping (ETL), and    e) a cathode.
Additional functional layers may be provided, such as charge carrier injection layers.
The emitting layer is usually established as a mixed layer of an emitter and a host material (blending, no electrical doping). For some realizations of high efficiency OLEDs or OLEDs emitting several portions of the visible light spectrum, e. g. white light emitting OLEDs, the emission zone can as well exist of even more layers, all of them electrically non-doped.
PIN OLEDs have been processed in VTE (VTE—Vacuum Thermal Evaporation), i. e. at least layers two to four are produced in VTE. Here, the materials are thermally evaporated from independent crucibles. Temperature is chosen to meet the evaporation point of the material at high vacuum conditions which is usually in the range from 100-400° C. The depositions rates of the materials are usually in the range of 0.01 to 0.5 nm/s, for high throughput deposition as well up to 5 nm/s. Pressure conditions used are usually <10−3 mbar, preferably <10−5 mbar.
In the state of the art, electrical doping of thin films is performed by VTE. There, the dopant and the host materials are evaporated simultaneously in different crucibles. The depositions rates of the materials in co-deposition are usually in the range of less than 0.01 to 5 nm/s. However, the deposition takes place at low vapour pressure of the materials. The evaporation cones of the crucibles are aligned towards the sample. In a co-deposition the cones of host and dopant superpose partially. Nevertheless, due to the low vapour pressure leading to a long mean free path for a molecule in the gas phase and the rather short shared path of the host and dopant, they do mainly mix in the growing layer. There, the doping process takes place by a chemical reaction of dopant and host material. Collisions and/or chemical reactions of the molecules in the gas phase are highly unlikely. Usually, with a doping concentration of 0.2 to 10 mol-% electrical conductivities of about 10−7 to 10−5 S/cm can be achieved compared to less than 10−8 S/cm for non-doped host material.
Organic vapour phase deposition (OVPD™) has been demonstrated to be an efficient and versatile means for the growth of organic electronic components based on small molecular weight materials that has many attributes that make it suitable for high volume production of devices (see M. A. Baldo et al., Appl. Phys. Lett. 71, 3033 (1997); M. A. Baldo et al., Adv. Mater. (Weinheim, Germany) 10, 1505 (1998); M. Shtein et al., J. Appl. Phys. 89, 1470 (2001)). The OVPD process is based on the sublimation of small molecular weight organic materials into an inert carrier gas stream in a hot walled chamber. The gas transports the molecules to a cooled substrate where they rapidly condense to form the desired film.
A number of organic electronic devices grown by OVPD have been reported, including organic light-emitting devices (OLEDs), thin film transistors, and photovoltaic cells (see M. Shtein et al., J. Appl. Phys. 89, 1470 (2001); M. Shtein et al., Appl. Phys. Lett. 81, 268 (2002); P. E. Burrows et al., J. Cryst. Growth 156, 91 (1995)).
Compared to the conventional technology of vacuum thermal evaporation, OVPD has potential advantages such as improved thickness uniformity, precision control of layer interfaces, elimination of parasitic deposition on the chamber walls (leading to high deposition efficiency and reduction in system maintenance), and of simultaneous deposition of multiple materials/components.
The multi-chamber tool usually consists of an OVPD system integrated with a vacuum transfer chamber, a VTE chamber for deposition of high evaporation temperature organics and cathode metals, and a nitrogen glove box. The OVPD system is comprised of remotely positioned source furnaces, gas manifolds to combine about ten organic sources into a single inert gas flow stream, and the deposition chamber. The organic materials are heated in independent source cells in the furnaces, thereby eliminating cross contamination. The organic vapour is transported from the source cells by preheated and purified nitrogen gas via heated lines/pipes (temperature above sublimation temperature of the organic material with highest sublimation temperature to be transported through pipe system) into the deposition chamber. The deposition chamber contains a showerhead vapour distributor located several centimeters above the substrate surface, which is cooled to a temperature in the range of 5 to 40° C. The deposition chamber pressure is at about 0.7 Torr, while the source cell pressure is at 7.5 Torr. This reflects the requirement for maintaining a nearly ten-fold differential in upstream pressure to achieve the desired deposition rates, while overcoming the line impedances due to the remote placement of the organic sources from the reactor vessel.
Since the source temperature is kept constant to within less than 1° C. of its set point, the deposition rate is primarily controlled by the carrier gas mass flow rate, thus eliminating the need for an in situ crystal thickness monitor which is required for VTE. Hence deposition rate is calibrated based on the reactor conditions, and thereafter the growth time is set to achieve the required thickness, which is checked post-growth, e. g. by ellipsometry. At a constant source temperature, the carrier gas flow rate Q through an individual container, determines the concentration of the organic vapour. The total flow is maintained at about 1000 sccm to ensure consistent spatial thickness uniformity. Each source container is then calibrated by obtaining the dependence of deposition rate r on Q.
Mixed layers can be deposited by OVPD. A source calibration is used to choose appropriate deposition rates and time for each component material in the organic electronic device. In particular, for the emission layer of an OLED, the host (also labelled matrix) and dopant materials are combined in the vapour phase in a small mixing volume placed at the input to the showerhead, thus enabling a homogeneous blending of the host and multiple dopant molecules before they reach the substrate. In other modifications of the OVPD system mixing of carrier gases is done in or before the line system at a certain distance from the showerhead. Since the deposition rate is calibrated by thickness measurements, the doping concentration is defined by the volume (thickness) ratio between the host and dopant material as opposed to the weight percentage conventionally used for VTE.
OVPD has been demonstrated for layers consisting of one material or several materials which are chemically inert (see T. X. Zhou et al., Appl. Phys. Lett. 86, 21107 (2005)). That means the materials are co-deposited as a blend of molecules which do not undergo a chemical reaction with each other. A typical example for such a co-deposition is the emittet doping in the emission layer of an OLED. Another example is the co-deposition of two materials either to improve the ability to transport both electrons and holes of the layer or the morphology or to increase glass-transition temperature.
The processing of layer growth in OVPD is different from the process using VTE: Here, the materials are heated in independent source cells. The organic vapour is transported from the source cells by preheated nitrogen gas via heated lines into the deposition chamber. The partial gas pressure of the materials is usually higher than in VTE and, more important, the vapour of dopant and host material are mixed in the line. Thus, dopant and host molecules have a higher interaction probability before the reach the substrate. As a result, they can undergo a chemical reaction before reaching the substrate and, depending on the material, will fall out or stick to the wall of the line and are not incorporated in the layer. In general, charge-transfer complexes or salts likely to be formed in such a reaction are known to be less volatile than the neutral components they have been formed from. This can be due to the high dipole moment or strong ionic bonding typical for such charge-transfer complexes or salts. Furthermore, due to frequent collisions with the walls and/or carrier gas molecules, the momentum of the organic molecules is not conserved. In consequence, a reduced conductivity is obtained for the doped layer in OVPD, in contrast to VTE.